Semiconductors are typically fabricated by depositing and etching a number of layers that are shaped and configured on the upper or top surface of a wafer. Controlling those fabrication steps and testing the wafer early during production helps to keep production costs low. An increasingly important technique for a non-destructive measurement of semiconductors is ellipsometry. In ellipsometry, a specifically configured probe light beam is directed to reflect off the wafer. The change in polarization state of the beam induced by the interaction with the wafer is monitored to provide information about the wafer.
Ellipsometers have been used extensively to monitor thin film parameters such as thickness, index of refraction and extinction coefficient. More recently, ellipsometers have been used to monitor the properties (critical dimensions) of small, repeating, periodic structures on wafers. These periodic structures are similar to a grating and the measured data can be subjected to a scatterometry analysis to derive information about the structure. Information of interest includes, but is not limited to, line width and spacing as well as sidewall profile.
Such periodic structures have distinct orientations. It has been found that the most useful information about such structures can be obtained if the probe beam of the ellipsometer is directed substantially perpendicular to the line structure.
As seen in FIG. 1, a typical wafer W will have multiple such periodic structures PS formed thereon. In some cases, all of the periodic structures will be oriented in the same direction (i.e. all lines parallel). In other cases, some of the structures will have lines running perpendicular to other structures.
A conventional, ellipsometer is typically provided with a stage for moving the wafer through full linear motions FX, FY as well as rotation about the central axis so that the probe beam PB can be directed to each of the periodic structures PS in the appropriate direction (usually perpendicular to the line structure). The linear motions FX, FY are about equal to the wafer diameter WD. The wafer W moved during the measurement consequently occupies a travel envelope LE that extends in the directions of each of the linear axes about twice the wafer diameter. The travel envelope LE determines the minimal footprint of an ellipsometer apparatus.
Recently, there has been a push to substantially reduce the size of ellipsometer apparatus. This effort is particularly directed to allowing an ellipsometer to be incorporated directly into a semiconductor processing tool. To achieve the desired miniaturization, stage system have been developed which reduce the total range of motion of the wafer, thereby reducing the travel envelope and consequently the footprint of the system (e.g. stages that implement a cylindrical coordinate system consisting of a linear and a rotational stage). The use of these stages has not significantly impeded the measurement of thin film parameters since such measurements are not effected by the direction in which the probe beam strikes the sample. However, such reduced motion stage systems have caused a problem with measuring periodic structures where the impinging direction of the probe beam PB has to correspond to a measurement relevant orientation of the periodic structure PS.
As shown in FIGS. 2A and 2B the wafer can be manipulated with the X- and Y-stages such that only one of the four quadrants of the sample is located at the intersection between the sample and the probing beam. To perform measurements within the other three quadrants of the sample, the rotating stage needs to move by multiples of 90°. To achieve perpendicular orientation of the periodic sample structures in these cases, a second probing beam perpendicular to the first one is necessary.
This difficulty can best be seen in FIGS. 2A and 2B. In FIG. 2A, the probe beam PB is shown striking periodic structure PSI perpendicular to the line structure. When the operator wishes to measure periodic structure PS2, the rotating stage is used to bring the sector of the wafer where that structure is located within the region which can be reached by the probe beam. As noted above, the stage travel in the X and Y directions is not sufficient to bring the structure PS2 under the probe beam without a rotation. Unfortunately, and as seen in FIG. 2B, the result of this rotation is to orient the periodic structure PS2 so that the probe beam impinges thereon in a direction parallel to the lines. As noted above, it has been found that most relevant information can be obtained when the beam strikes the structure perpendicular to the line structure.
Accordingly, it would be desirable to develop an ellipsometer system, which can utilize a reduced motion stage but also provides for optimal measurement of both thin film parameters and periodic structures.